With an annual production of twelve billion tons, concrete has emerged as the material of choice for modern infrastructure construction. Portland cement concrete, a mineral-based material, is a composite material produced from proportioned mixtures of water, Portland cement, fine and coarse aggregates, and in some cases, mineral admixtures, chemical admixtures, and fibers. The products of cement hydration bind the aggregate particles together. Inherent in the microstructure of concrete are voids and cracks, ranging in size from microscale to macroscale. These voids and cracks can influence the mechanical behavior of the product. The presence of such flaws coupled with the generally brittle nature of concrete cause the tensile strength of concrete to be only ⅛ to 1/12 its compressive strength.
In order to enhance the properties, fibers have been used in various cement-based materials, including mortar, concrete, and fiber-cement products. Fibers conventionally used in concrete construction generally range from 30-60 mm in length. While randomly distributed fibers of this size can improve certain mechanical properties of concrete, mixing and dispersion are typically more demanding because of the likelihood of fiber “balling” or flocculation and non-uniform dispersion. Finishing is also challenging because “fuzzing” of fibers at the concrete surface can occur. These difficulties in mixing and dispersing the fiber limit the fiber volume that may be practically incorporated in concrete to 2% or less by volume. In addition, to non-uniform dispersion, steel, glass, and synthetic polymer fibers may increase the cost of concrete by 10-50%, rendering their use less cost effective.
Another source of fiber used for reinforcing mineral-based materials is asbestos fibers. However, the use of these fibers is hazardous and requires the provision of safety measures. In addition, due to their nature, asbestos fibers have a tendency to form clusters and bundles. For these reasons, asbestos have generally been replaced by other wood-based fibers.
Wood-based fibers, which range from 1 to 4 mm in length and are tens of microns in diameter, are similar to other types of microfibers previously investigated for concrete reinforcement. Also the tensile strength and elastic modulus of microfibers vary with material type. Wood pulp fibers were found to be comparable to the other types of fibers. The hydrophilic surfaces of cellulose fibers may facilitate their dispersion and bonding to the cement paste. Since fibers have a very large aspect ratio of length to width, and they are very flexible, the fibers tend to be entangled forming flocs, which are very difficult to break up into individual fibers. The fibers in the dry pulp are bonded strongly by hydrogen bonds. When the dry pulp is wetted, water breaks most of the hydrogen bonds, however mechanical entanglement still remains and makes the dispersion of the fibers very difficult. Strong shear force is required to disperse the entangled fibers. Therefore, if well dispersed, the relatively high surface area and the close spacing of pulp fibers, when combined with their desirable mechanical characteristics, make them effective in suppression and stabilization of microcracks in a concrete matrix.
The dispersion of pulp fibers are effective when used in small quantities or volume fractions, like 0.1% or less, but higher fiber contents may cause the balling and poor distribution throughout the cement paste matrix. Rather than improving the properties of a mineral-based material, fibers which are not well distributed through the cement paste matrix act instead as defects, creating points of weakness in the composite. Balling or clumping of poorly distributed fibers may cause a decrease in the mechanical characteristics and the durability of the bulk composite.
A major challenge to the introduction of pulp fibers to the cement-based matrix is the uniform dispersion of the fibers throughout the matrix. The pulp fibers in their original form are very difficult to disperse in an inorganic binder, such as a cement paste matrix.
The Inventors have developed a method for effectively reinforced mineral-based materials. The Inventors have developed a method that improves the dispersion of reinforcing fibers into the mineral-based materials. The Inventors have further developed a low-cost method, to improve dispersion and bonding of fibers in mineral-based materials, resulting in composite materials with enhanced performance for a variety of applications such reduction of early age cracking and reduction of shrinkage cracking.